Based on the evidence that fixed (licensed) spectrum allocation entails a highly inefficient resource utilization, cognitive radio prescribes the coexistence of licensed (or primary) and unlicensed (secondary or cognitive) radio nodes on the same bandwidth. While the first group is allowed to access the spectrum any time, the second seeks opportunities for transmission by exploiting the idle periods of primary nodes, as described in more detail in S. Haykin, “Cognitive radio: brain-empowered wireless communications,” IEEE Journal on Selected Areas Commun., vol. 23, no. 2, pp. 201-220, February 2005, the entire content of which is incorporated herein by reference. The main requirement is that the activity of secondary nodes should be “transparent” to the primary, so as not to interfere with the licensed use of the spectrum.
Centralized and decentralized protocols at the media access control (MAC) layer that enforce this constraint have been studied in Y. Chen, Q. Zhao and A. Swami, “Joint design and separation principle for opportunistic spectrum access,” in Proc. Asilomar Conf. on Signals, Systems and Computers, 2006, and Y. Xing, R. Chandramouli, S. Mangold and S. Shankar N, “Dynamic spectrum access in open spectrum wireless networks,” IEEE Journ. Selected Areas Commun., pp. 626-637, vol. 24, no. 3, March 2006, the entire contents of which are incorporated herein by reference. Here, the radio channel is modeled as either busy (i.e., the primary user is active) or available (i.e., the primary user is idle) according to a Markov chain. Information theoretic study of cognitive radios at the physical layer that take into account the asymmetry between primary and secondary users are presented in: N. Devroye, P. Mitran and V. Tarokh, “Achievable rates in cognitive radio,” IEEE Trans. Inform. Theory, vol. 52, no. 5, pp. 1813-1827, May 2006; A. Jovicic and P. Viswanath, “Cognitive radio: an information-theoretic perspective,” available on-line at http://lanl.arxiv.org/PS_cache/cs/pdf/0604/0604107.pdf; S. A. Jafar and S. Srinivasa, “Capacity limits of cognitive radio with distributed and dynamic spectral activity,” preprint [http://arxiv.org/abs/cs.IT/0509077]; and Kyounghwan Lee and A. Yener, “On the achievable rate of three-node cognitive hybrid wireless networks,” in Proc. International Conference on Wireless Networks, Communications and Mobile Computing, vol. 2, pp. 1313-1318, 2005, the entire contents of all of which are incorporated herein by reference. Alternatively, game theory has been advocated as an appropriate framework to study competitive spectrum access in cognitive networks in J. Neel, J. Reed, R. Gilles, “The Role of Game Theory in the Analysis of Software Radio Networks,” in Proc. SDR Forum Technical Conference, 2002, the entire content of which is incorporated herein by reference. Finally, the concept of cognitive radio has been embraced by the IEEE 802.22 Working Group, that is working towards the definition of a Wireless Regional Area Network standard for secondary use of the spectrum that is currently allocated to television service, as described in more detail in C. Cordeiro, K. Challapali, D. Birru and Sai Shankar N, “IEEE 802.22: the first worldwide wireless standard based on cognitive radio”, in Proc. IEEE DySPAN, pp. 328-337, 2005, the entire content of which is incorporated herein by reference.
A cognitive network where two source-destination links, a primary link and a secondary link, share the same spectral resource (for example, a cognitive interference channel, as shown in FIG. 1) has been recently investigated in the landmark paper by Devroye et al. and in Jovicic et al. from an information theoretic standpoint. In these references, a cognitive transmitter is assumed to have perfect prior information about the signal transmitted by a primary transmitter (see also P. Mitran, N. Devroye and V. Tarokh, “On compound channels with side information at the transmitter,” IEEE Trans. Inform. Theory, vol. 52, no. 4, pp. 1745-1755, April 2006, the entire content of which is incorporated herein by reference). However, imperfect information on the radio environment (e.g., on the primary activity) at the cognitive transmitter (or node) is expected to be a major impediment to the implementation of the cognitive principle, as described in more detail in A. Sahai, N. Hoven and R. Tandra, “Some fundamental limits on cognitive radio,” in Proc. Allerton Conference on Communication, Control, and Computing, October 2004, the entire content of which is incorporated herein by reference. Moreover, traffic dynamics at the primary are of great importance in defining the performance of cognitive radio, but random packet arrival cannot be easily incorporated in a purely information theoretic analysis.